The invention relates to a string binder for use in reinforced composite molding applications, and composite articles formed therefrom. Specifically, the novel string binder of the present invention comprises solid strands of a resin composition comprising one or more modified binder resins of low acid value, and, optionally, a fibrous carrier substrate material. The string binder preferably comprises at least one thermoformable resin as the binder resin component, and an effective amount of a catalyst having a high activation temperature. The string binder strands may be combined with one or more fibrous reinforcing materials to form a multi-end roving product, which may be used either in chopped or continuous form as a reinforcement material having improved impact strength. Such reinforcement materials are useful in numerous reinforced composite applications, including the molding of preforms typically used in liquid resin molding of fiber-reinforced articles. The invention further comprises a method of making the novel string binders of the invention.
Reinforcing fibers comprising glass, polymer, other reinforcing fibers, or blends thereof are commonly used as reinforcement materials in molded plastic composite articles. These reinforcing materials, when incorporated into the matrix resin of the composites, provide the finished product with a higher level of tensile strength and durability than could possibly be achieved if either the fibers or the resins were used separately. Reinforcing fibers may be incorporated into a composite resin matrix either in continuous form, as is done in the manufacture of filament-wound composites, or the fibers may be introduced into the matrix as chopped segments that may be dispersed throughout the matrix in linear or random fashion, depending on the characteristics that are desired in the final product.
Generally, in the manufacture of reinforcing articles for use in liquid resin molding processes, chopped segments of a fibrous substrate, typically glass strands, may be combined with a binder resin and the resulting composition is laid down over a form and solidified to form a matted structure such as a preform, which can then be cured and/or subjected to further molding processes to form the composite end product.
Several means of combining the binder resin with a glass carrier substrate strand to make preforms are known in the art. For example, an emulsion comprising a heat-curable binder resin may be blended with the glass carrier strand; or the resin and the carrier strand may be combined to form a slurry. The emulsion or the slurry may then be poured onto a form or mold and suction or a vacuum applied to remove the diluent or solvent component, thereby solidifying the preform. The obvious drawbacks associated with using an emulsion binder include the requirement for extensive clean-up of the forming screens; the environmental hazards relating to the discharge of solvent or diluent vapors containing volatile organic chemicals (VOCs); risks to the safety of personnel from exposure to such chemicals; and added costs arising from a lengthy drying period or the need for additional equipment to prepare the preform.
Dry compositions using, for example, a powdered binder in combination with the fibrous carrier material are also known. The powdered binder is heated sufficiently to melt and cure the binder after it is combined with the carrier material. One disadvantage of using the powdered binder is that it may be difficult to control the amount of binder powder required to create an acceptable preform, and the addition of excess resin may foul equipment and require extensive cleanup operations.
To make a preform using molten binder, typically, a glass fibrous carrier substrate is chopped into segments, which are combined with the binder resin and placed over a porous structural form such as a mesh screen. Alternatively, the glass carrier substrate material may be formed into strands that are then chopped into segments and sprayed over the form in combination with a binder. The method of adding the binder may be via a flame-spray process, in which solid, powdered binder resin is sprayed through a flame immediately before it contacts the fibrous carrier material. In this fashion, the binder is melted before it mixes with the fibrous carrier. A process involving the steps of heating, curing and cooling of the material is then applied to form, shape and consolidate the material, as well as to remove any solvents or diluents that may be present, thereby solidifying the product into a preform ready for molding or further processing. The resulting preform may then be removed and used in a subsequent molding operation, such as injection molding, in which a resin is injected around the preform and cured to form a structurally molded composite.
Because these techniques of making the preform typically require applying an excess of binder resin, a commonly observed drawback is the build-up of excess molten binder resin on the equipment, the removal of which is both costly and time-consuming. Moreover, the process includes the inherent difficulties of dealing with the molten binder. The process of adding the binder is difficult to control, and the handling of the molten resin poses an additional safety concern.
Continuous glass fibers that have been pre-impregnated with a binder resin may also be used to form fiber segments for preform manufacture. The impregnated strands, known as string binders, may be formed by applying one or more layers of a binder resin onto the surface of a continuous glass fiber strand after it is formed, then allowing the binder to set on the surface of the strand. After the coating is solidified, the strand is then chopped into coated segments that may be used in the spray-up process to make preforms.
The binders used in preform manufacture are usually either thermoplastic polymers in molten or powdered form, or high acid value thermoset emulsion polymers such as crystalline polyesters. The term xe2x80x9ccrystallinexe2x80x9d relates to the inherent ability of the thermosetting resin to form crystallites or regions of order dispersed among regions of disorder within the solidified polymer. The ability of a polymer to display crystalline properties is determined principally by its composition. For example, thermoplastic polyesters are macromolecules that contain no chemical groups to effect inter-linking. Such materials are typically heated to the softening point, forced into the shape of the desired article, then cooled below the softening point to yield the finished reinforcing article. Like thermosetting polyesters, they may display many levels of crystallinity, again depending on composition. Crystalline polyesters find use in organic fiber manufacture. Perhaps the best known crystalline polyester is polyethylene terephthalate, PET, which is commonly known as DACRON polyester, available from DuPont Inc.
The term xe2x80x9chigh acid valuexe2x80x9d, as used herein, is intended to represent the acidity of the polymer in terms of the amount of potassium hydroxide (KOH) required to neutralize the acidic functional groups in one gram of the polymer. A high acid polymer is one that contains acidic functional groups such that the measured acid value of the polymer is greater than 30 mg KOH/g of polymer. The known drawbacks of using the above high acid polymers include a high level of incompatibility between the binder resin molecules and the composite matrix resin because of the large degree of difference in polarity between the binder polymer molecules and the matrix resin molecules and/or the absence or unavailability of reactive functional groups that can cross-link with the composite matrix resin. This incompatibility can result in a lesser degree of wet-out of the reinforcing fibers in the composite matrix resin, and associated product defects such as blistering during the composite molding phase, and bleeding or blistering during post-bake of the composite product.
Bleeding is related to certain characteristics of the binder resin that affect compatibility with the matrix composite resin. While thermoplastic and thermosetting resins have been used as a binder resin in string binder formulations, the different characteristics of these types of polymers affect their use in composite formulations. Where the binder resin is a thermosetting polymer, a resin with a lower molecular weight may generally be used because the molecules will link during cure to form a permanently solidified, continuous, cured matrix with essentially infinite molecular weight. The lower molecular weight resin will easily flow and therefore will more fully coat the fibers of the fibrous substrate. Typically, such binder resin polymers are thermosetting crystalline polyester resins made up of small molecules, which melt and flow easily. In contrast, molecules of thermoplastic resin do not link to form a permanently solidified matrix. Rather, the solidified matrix may be induced to re-melt and flow by applying heat. In order to achieve acceptable performance using a thermoplastic resin, it is typically necessary to begin with resins that have a higher molecular weight. Such compounds are usually composed of long chains of atoms, which become easily entangled, thereby causing a restriction of flow. This reduced flow, which results in a higher melt viscosity, is a disadvantage in that it impedes flow of the coating over the fibers. Further, the large, unlinked thermoplastic molecules demonstrate a tendency to diffuse through the composite matrix during post-baking. This diffusion or bleeding typically causes blemishes in the surface of the composite.
Blistering may result from an undesirable chemical reaction between a component of the composite matrix resin and the binder resin during the composite curing process. For example, where the composite matrix resin is a polyurethane, an isocyanate group of the polyurethane may react with acid or water in the binder to form carbon dioxide and an amine as reaction by-products. The evolution of the carbon dioxide gas can lead to the formation of blisters on the surface of the cured composite. Blistering may ultimately result in decreased glass/matrix resin bond strength in the preform-reinforced composite, and, as a result, the physical strength of the finished molded article may be diminished. Blistering is also aesthetically undesirable because the appearance of the finished product is compromised.
There is, therefore, a need for a fibrous carrier substrate material that is efficiently combined with a binder resin before the preform is molded, such that the separate application of a liquid binder in the form of a powder melt, emulsion or slurry is not required. Further, there exists a need for a fibrous reinforcement material in combination with a binder resin that enhances wet-out, and prevents undesirable effects such as blistering or bleeding when the binder-coated fibers are used during the composite formation process. There also exists a need for a preform composition material that does not rely on the use of organic solvents that are environmentally hazardous, or other solvents that require a drying procedure that lengthens the manufacturing process. These needs are met by the invention described herein.
This invention relates to a string binder comprising polymerized filaments of a thermoformable resin material having a low acid value, preferably less than about 30 mg KOH/g of resin. The string binder may further comprise a fibrous carrier substrate material coated with a composition comprising a thermoformable binder resin material, said composition imparting thermoformability to the fibrous carrier material used in reinforcing articles made using the string binder. The string binder of the present invention may optionally be co-roved with one or more strands of at least one other reinforcing material to form a multi-end roving for use in manufacturing reinforcing articles such as preforms, which may be used in an injection molding process. Further, the string binder and the co-roved product containing it may be formulated without the use of a liquid emulsion binder. Instead, an effective amount of a binder resin coating composition may be applied to the surface of the carrier substrate material and solidified to form a ready-to-use product comprising the binder resin and the fibrous carrier material. The invention further relates to a composition for forming a reinforcing article comprising a thermoformable binder resin having a low acid value.
As used herein, the term xe2x80x9cthermoformablexe2x80x9d is intended to mean a resin that can be formed by heating, such as a thermoplastic, or a resin that is irreversibly set using heat, such as a thermosetting resin. The binder resin comprises a polymer that is specially modified to have a low acid value, preferably in the range of less than about 30 mg KOH/g resin, and most preferably less than 10 mg KOH/g resin. The term xe2x80x9cfibrous carrier materialxe2x80x9d is defined to mean a fibrous substrate selected from reinforcing materials that are commonly known in the art. The xe2x80x9cbinder resin materialxe2x80x9d is a polymer that is used to fuse the fibers or strands of the fibrous carrier material such that the mixture of fibrous carrier and the binder resin may be solidified and cured to form a reinforcing article such as a preform, which may be used in a further manufacturing process to make a composite article. Where the reinforcing article is a preform, it may typically comprise from about 10% to about 15% by weight of string binder, with from about 85% to about 90% by weight of another reinforcing material. The ratio of the amount of fibrous carrier material to the amount of binder resin material is preferably about 50:50 in the string binder.
The string binder of the present invention preferably combines the fibrous carrier substrate material with the binder resin composition to form a solid product that may be used in continuous or chopped form as a raw material in the preparation of preforms for molding processes. In such an embodiment, the product comprises a binder resin coating material solidified on at least a portion of the fibrous carrier material of the string binder. Additionally, other embodiments of this product may include, for example, a string binder comprising the novel binder resin material of this invention in the absence of a carrier substrate. Further, the string binder of either of the previously described embodiments may be combined with a reinforcing material in addition to that used as the fibrous carrier, in an amount sufficient to form a reinforcing article. As used herein, the term xe2x80x9cfibrous reinforcing materialxe2x80x9d is a material generally known in the art for providing reinforcement, which is used in addition to the fibrous carrier substrate material. This material is preferably in the form of a continuous roving having linearly aligned filaments.
Compared to the conventional approach of applying a liquid binder to strand segments during preform manufacture, there are several known advantages to using the various embodiments of the string binder of the present invention. For example, where the string binder is desirably used to make a reinforcing article, use of the string binder as a reinforcement is greatly simplified because the need to apply a liquid binder resin on the preform screen is eliminated. Specifically, the problems of poor binder resin application efficiency and excess build-up of resin on equipment, which has been a concern of preform makers using more conventional approaches, is altogether eliminated. Additionally, because no solvents are used to dissolve or emulsify the binder resin, emissions of volatile organic chemicals from the solvent during the forming operation are substantially eliminated, and the associated risks to worker safety are removed. Use of the string binder product of the invention also results in products having superior physical characteristics and appearance.
In accordance with yet another aspect of this invention, the string binder may comprise a core strand of a fibrous carrier substrate, and a binder resin coating further comprising a catalyst. The catalyst may be applied as a separate layer in addition to the binder-comprising coating of this invention, or, alternatively, it may be incorporated into the binder resin coating composition before it is applied to the surface of the reinforcing fibers. The role of the catalyst is to effect cure of the binder resin during the manufacture of the reinforcing article. Accordingly, the catalyst must be a compound or mixture of compounds that is compatible with the binder resin and which has a temperature of activation that is higher than the processing temperature used to melt the string binder as it is being formed into the reinforcing article. For example, where the reinforcing article is a preform, the catalyst should have an activation temperature that is higher than the temperature required to melt the binder resin as the string binder is formed into the preform. In this manner, the binder resin polymer may, for example, be used to coat the fibrous carrier material, or it may be formed alone as a string binder, without initiating the curing process. Rather, the activation temperature of the catalyst is such that catalytic activity is initiated during the step of curing of the preform.
Still another aspect of the invention includes a method for making a string binder, comprising the steps of:
a) forming a strand of a fibrous carrier substrate;
b) preparing a solvent-free binder coating composition comprising a thermoformable liquid binder resin material having an acid value of less than about 30 mg KOH/g of resin;
c) applying the binder coating composition to the surface of the fibrous carrier substrate material and allowing it to set into a solid coating, thereby forming a string binder; and
d) optionally chopping the string binder into segments.
The string binder may optionally be co-roved with one or more strands of a fibrous reinforcing material to form a multi-end roving which may be used, in continuous or chopped form, in various reinforcing applications. Additionally, the string binder may be incorporated into a woven or stitched fabric reinforcement, such as a woven roving or a multi-axial stitched reinforcement.
The inventive concept herein disclosed also includes a preform manufactured using a chopped segments of a thermosettable string binder as is herein described, or chopped segments of a multi-end roving comprising one or more strands of the string binder in combination with one or more strands of a fibrous reinforcing material. The chopped segments may be laid up on a consolidation screen, and optionally compressed using suction drawn through the screen to form the material into a desired shape that conforms to the contour of the screen. Preferably, the binder resin in the preform is fully cured before the preform is molded to form the composite product.
In yet another embodiment, the invention provides a reinforced, molded article formed by molding a moldable material comprising a matrix polymer such as a thermoplastic or thermoset polymer, in contact with a preform comprising the string binder which is herein described.
These and other attributes of the invention will be clear to those skilled in the art in light of the following description and drawings.